Hydrofluoplumbic Acid, H4PbF8

Hydrofluoplumbic Acid, H4PbF8, is probably formed in solution when lead tetra-acetate is dissolved in concentrated hydrofluoric acid. When this solution is evaporated, lead dioxide separates (Brauner). The potassium salt K3HPbF8 was prepared by Brauner by acting on the freshly precipitated oxide Pb5O7.3H2O with a solution of potassium hydrogen fluoride in aqueous hydrofluoric acid, by fusing together lead dioxide and potassium fluoride and mixing hydrofluoric acid with the product, and by bringing together lead tetra-acetate potassium hydrogen fluoride and hydrofluoric acid. The solution obtained in either of these ways is evaporated in a vacuum desiccator, when monoclinic needles of the required salt are obtained. These are isomorphous with the corresponding tin salt K3HSnF8. The salt is stable in dry air, but is decomposed by water with separation of hydrogen fluoride, thus:

K3HPbF8 + 2H2O = 3KF + PbO2 + 5HF.

At 230° C. hydrogen fluoride is evolved, and at a higher temperature fluorine, thus:

K3HPbF8 → K3PbF7 → 3KF + PbF2 + F2.

It is noteworthy that this reaction furnishes a chemical method of preparing fluorine.